513: Should we care about fetal growth percentiles at 18-22 weeks?

To determine the likelihood of small-for-gestational age at birth of anatomically normal fetuses identified as having fetal growth restriction (FGR) at their 18-22 week anatomy ultrasound (US). We performed a retrospective cohort study of deliveries at our institution from January 1, 2013 to March 31, 2017. We included well-dated singleton infants who were anatomically normal and had an ultrasound at 18-22 weeks followed by additional growth ultrasounds prior to delivery. Estimated fetal weights (EFWs) were calculated and percentiles assigned using the Hadlock formula and curve. Small-for-gestational age (SGA) was defined as birth weight <10th percentile. The primary outcome was the proportion of growth restricted fetuses at 18-22 weeks whose FGR was “un-diagnosed” by an EFW >10th percentile on a subsequent exam. Secondary outcomes were mean gestational age (GA) at delivery, as well as the sensitivity, specificity, positive and negative predictive values to predict SGA at birth. We identified 1226 patients who had multiple US exams, including one at 18-22 weeks, with a mean of 3.14 exams per patient. The rate of SGA at birth was 11.1%. Thirty-seven (3%) fetuses had FGR at 18-22 weeks, with 18 (49%) of those ultimately being “un-diagnosed” on a later exam and 29 (78%) ultimately having a normal birth weight. Among these fetuses with FGR at 18-22 weeks, the mean GA at delivery was 38.9 +2.4 weeks, with only 5 (14%) requiring delivery before 37 weeks. Of fetuses initially identified as FGR who were later undiagnosed (n=18), none were SGA at birth, while 42% of those who kept the diagnosis of FGR were SGA at birth, a difference that was statistically significant when compared with the rate of SGA (10.8%) among fetuses who were appropriately grown at 18-22w (p<0.001). Anatomically normal fetuses diagnosed with FGR at their 18-22 week anatomy US are likely to either have a subsequent EFW or birth weight >10th percentile. When a subsequent US demonstrates EFW >10th percentile, surveillance can be discontinued. However, when FGR persists, the rate of SGA is high, and antenatal surveillance is justified.